1. Field of the Invention
The present invention relates to a control system and method for an internal combustion engine and an engine control unit which control an air-fuel ratio of a mixture to be supplied to cylinders by switching the air-fuel ratio between a leaner side and a richer side.
2. Description of the Related Art
Conventionally, as a control system of this kind, one disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2003-90250 is known. In this control system, an air-fuel ratio mode is switched between a lean mode that controls the air-fuel ratio of a mixture to a leaner value than the stoichiometric air-fuel ratio, and a rich mode that controls the air-fuel ratio to a richer value than the stoichiometric air-fuel ratio. In the lean mode, a target air-fuel ratio is set according to a target engine torque and the rotational speed of the engine, and a fresh air amount and a fuel injection amount are controlled such that the air-fuel ratio becomes equal to the set target air-fuel ratio. Further, target fuel injection timing is calculated according to the target engine torque and the rotational speed of the engine, and fuel injection timing is controlled based on the calculated target fuel injection timing.
On the other hand, in the rich mode, the target air-fuel ratio is set to a predetermined value. Further, the target fuel injection timing is set to a value obtained by adding a predetermined advanced value to the target fuel injection timing in the lean mode, which is calculated immediately before the air-fuel ratio mode is switched to the rich mode.
Further, when the air-fuel ratio mode has been switched, the target air-fuel ratio is finally calculated by performing predetermined delay processing (weighted average processing) on the target air-fuel ratio calculated as above for the lean or rich mode to which the air-fuel ratio mode has been switched. Similarly, the target fuel injection timing as well is subjected to predetermined delay processing upon switching of the air-fuel ratio mode.
As described above, when the air-fuel ratio mode has been switched between the lean mode and the rich mode which are very different in the air-fuel ratio, a fresh air amount suitable for the rich or lean mode to which the air-fuel ratio mode has been switched is supplied in a delayed manner, and the degree of delay of the fresh air amount is not necessarily uniform since it is changed according to operating conditions of the engine. However, in the above-described conventional control system, when the air-fuel ratio mode has been switched, uniform delay processing set in advance is simply performed on the target fuel injection timing, and therefore fuel injection timing is sometimes set in a manner deviated from the optimum value corresponding to the actual fresh air amount, which causes an increase in torque variation.